Add constexpr template specializations for built in metatypes.

This will make it possible (in Qt 6) to remove the enums listing
metatype ids. As it is constexpr, it can be used in switch statements
just like enums, as enum values, and as template specialization values.

Change-Id: I51293674c403714e34cb8a8b8953522fc97a740a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

diff --git a/src/corelib/kernel/qmetatype.h b/src/corelib/kernel/qmetatype.h
index a42333f..79514c8 100644 (file)
--- a/src/corelib/kernel/qmetatype.h
+++ b/src/corelib/kernel/qmetatype.h
@@ -536,17 +536,17 @@ template <typename T>
 struct QMetaTypeId2
 {
     enum { Defined = QMetaTypeId<T>::Defined };
-    static inline int qt_metatype_id() { return QMetaTypeId<T>::qt_metatype_id(); }
+    static inline Q_DECL_CONSTEXPR int qt_metatype_id() { return QMetaTypeId<T>::qt_metatype_id(); }
 };
 
 namespace QtPrivate {
     template <typename T, bool Defined = QMetaTypeId2<T>::Defined>
     struct QMetaTypeIdHelper {
-        static inline int qt_metatype_id()
+        static inline Q_DECL_CONSTEXPR int qt_metatype_id()
         { return QMetaTypeId2<T>::qt_metatype_id(); }
     };
     template <typename T> struct QMetaTypeIdHelper<T, false> {
-        static inline int qt_metatype_id()
+        static inline Q_DECL_CONSTEXPR int qt_metatype_id()
         { return -1; }
     };
 
@@ -624,7 +624,7 @@ void qRegisterMetaTypeStreamOperators(const char *typeName
 #endif // QT_NO_DATASTREAM
 
 template <typename T>
-inline int qMetaTypeId(
+inline Q_DECL_CONSTEXPR int qMetaTypeId(
 #ifndef qdoc
     T * /* dummy */ = 0
 #endif
@@ -713,7 +713,7 @@ inline int qRegisterMetaTypeStreamOperators()
     template<> struct QMetaTypeId2<TYPE> \
     { \
         enum { Defined = 1, MetaType = QMetaType::NAME }; \
-        static inline int qt_metatype_id() { return QMetaType::NAME; } \
+        static inline Q_DECL_CONSTEXPR int qt_metatype_id() { return QMetaType::NAME; } \
     }; \
     QT_END_NAMESPACE
 

diff --git a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
index b616257..c73c26a 100644 (file)
--- a/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
+++ b/tests/auto/corelib/kernel/qmetatype/tst_qmetatype.cpp
@@ -107,6 +107,7 @@ private slots:
     void saveAndLoadBuiltin();
     void saveAndLoadCustom();
     void metaObject();
+    void constexprMetaTypeIds();
 };
 
 struct Foo { int i; };
@@ -1632,6 +1633,46 @@ void tst_QMetaType::metaObject()
     QCOMPARE(QMetaType(QMetaType::Int).metaObject(), static_cast<const QMetaObject *>(0));
 }
 
+#define METATYPE_ID_FUNCTION(Type, MetaTypeId, Name) \
+  case ::qMetaTypeId< Name >(): metaType = MetaTypeIdStruct<MetaTypeId>::Value;
+
+template<int>
+struct MetaTypeIdStruct
+{
+};
+
+#define METATYPE_ID_STRUCT(Type, MetaTypeId, Name) \
+template<> \
+struct MetaTypeIdStruct< ::qMetaTypeId< Name >()> \
+{ \
+    enum { Value = ::qMetaTypeId< Name >() }; \
+};
+
+#if defined(Q_COMPILER_CONSTEXPR)
+QT_FOR_EACH_STATIC_TYPE(METATYPE_ID_STRUCT)
+
+template<int i = ::qMetaTypeId<int>()>
+struct MetaTypeIdStructDefaultTemplateValue
+{
+  enum { Value };
+};
+#endif
+
+void tst_QMetaType::constexprMetaTypeIds()
+{
+    int id = 0;
+    int metaType;
+
+    switch(id) {
+#if defined(Q_COMPILER_CONSTEXPR)
+      QT_FOR_EACH_STATIC_TYPE(METATYPE_ID_FUNCTION)
+      metaType = MetaTypeIdStructDefaultTemplateValue<>::Value;
+#endif
+    default:;
+    }
+    Q_UNUSED(metaType);
+}
+
 // Compile-time test, it should be possible to register function pointer types
 class Undefined;